In optical transport networks, it is desirable to provide information regarding presence or absence of optical signal at each repeater site through a LOS detection unit for network management purpose. A LOS can also occur due to a fiber cut in the fiber span prior to the repeater. One prior art approach to LOS detection (see for example, U.S. Pat. No. 6,075,629 issued to Al-Salameh et al., June 2000) is based on direct measurement of total signal power and total noise power within an amplification band, and comparing the measured signal power and noise power with an expected threshold noise power within the amplification band. This approach becomes inaccurate when the signal power becomes substantially smaller than the noise power, for example in a transmission link where transmission loss in a fiber span is high or signal launch power is low, such that optical signal to noise ratio (OSNR) is low.
Another prior art approach (see for example, U.S. Pat. No. 6,266,168 issued to Denkin et al., July 2001) uses an interference filter for improving the sensitivity of LOS detection in an optical protection switch in a bidirectional optical communication node. More specifically, the interference filter filters signals received over the service and protection paths to generate a main signal and a complementary signal for each of the paths.